Agronomists and botanists have recognized the vital function of sixteen nutrients needed by growing plants including the trace elements or micronutrients--zinc, chlorine, iron, manganese, copper, molybdynum and boron. It is also known that the optimal spectrum and concentration of micronutrients in a particular soil can vary depending on the plants being grown, soil properties, climate, and the stage of the plant growth cycle.
While most soils contain micronutrients at least to some extent and the overall need is small, depletion can occur with intensive agricultural activity. Even when the soil concentration is putatively adequate, other factors can prevent micronutrient uptake by the plant. Since micronutrients must be available as soluble ions, such ions can be immobilized in low solubility alkaline soils and/or can be tapped in clays or other organic materials as insoluble complexes.
It has been common practice to supplement phosphorus-impoverished soil by using a mineral fertilizer such as rock phosphate or apatite. Such minerals, however, do not supply the required micronutrients and can contain toxic elements such as fluorine and cadmium. Other minerals can be used to supply micronutrients, however, micronutrient content can not usually be adjusted.
Required micronutrients can also be supplied directly in bulk. One disadvantage of multiple mineral and/or inorganic phases is the varying rates at which the materials dissolve and diffuse in the soil. Also, such additions give little control over micronutrient soil concentration with respect to plant needs from day to day. Another disadvantage is the relatively large quantities needed to overcome solubility and soil impediments. Bulk fertilizer can be carried off by rain or irrigation water or leached from the soil as a ground water pollutant. Thus, bulk use is inefficient and costly.
In space travel, agronomical species such as wheat, potatoes, soybeans, and the like in hydroponic tanks can fixate carbon dioxide into edible biomass. Hydroponic systems, however, require soluble nutrients to be continuously supplied and closely monitored. Hence, solid-support substrates may provide a low maintenance, high reliability substrate for plant growth. It is, therefore, very desirable that the fertilizer is strictly tailored to the requirements of the on-board cultures. In addition, the fertilizer should be compact, long-lasting and contain all required micronutrients, which are released as needed over a long period time, encompassing several growing seasons, without replenishment.
It is consequently desirable to tailor a synthetic phosphate fertilizer composition to specific agronomic needs in both terrestrial and space environments. It is also desirable that the composition have only the desired spectrum of micronutrient elements which can be released in a controlled manner and be free of toxic elements which are normally present in natural phosphate fertilizers.
U.S. Pat. No. 3,958,973 to Roberts describes a micronutrient metal containing phosphate glass for fertilizer use. The glass is based on P.sub.2 O.sub.5 and the micronutrients are based on the metal oxide. A solubility control agent is said to be present to control the amount and rate of release.
U.S. Pat. No. 4,299,613 to Carderelli describes a polymeric composition incorporating essential plant growth compounds in ionic form. These compounds are said to be gradually, continuously and uniformly released over a long period of time in response to the presence of moisture.
U.S. Pat. No. 4,334,906 to Young describes a combination soil amendment and micronutrient source. The composition comprises highly porous sulfur particles having substantial internal surface area with the micronutrient source dispersed either throughout the particle matrix or over the interior surfaces.
U.S. Pat. No. 4,670,039 to Sjogren describes a controlled slow release fertilizer composition comprising an encapsulated fertilizer, carbon particles and plaster.
U.S. Pat. No. 4,994,100 to Sutton et al. describes a granular nitrogen fertilizer made up of urea, dicyandiamide, ammonium thiosulfate, and optionally a phosphate compound. This composition can contain various micronutrients.
U.S. Patent 4,557,749 to Berthet et al. describes a sealed container for a hydrosoluble fertilizer or agricultural product. The container has a wall made of a hydrophobic polymer diaphragm with hydrophilic inclusions which absorb water. Water passing into the container dissolves the fertilizer which is then desorbed into the medium being treated.
Other U.S. Patents of interest include U.S. Pat. Nos. 4,507,139 to Sullivan et al.; 4,175,943 to Jordaan et al.; and 4,995,897 to Schramm et al.